The radio-controlled transmission of time information is performed by transmitting so-called time signals by respective transmitters referred to herein as time signal transmitters or simply transmitter. The term “time signal” is intended to mean a transmitter signal of short duration for providing time information in the form of a time reference provided by the transmitter. The time reference is a modulated oscillation generally comprising several time markers which, upon demodulation, merely are an impulse which reproduces the transmitted time reference with a determined uncertainty or inaccuracy.
The German long wave time signal transmitter station DCF-77 transmits, in a continuous operation controlled by atomic clocks, amplitude modulated long wave time signals in accordance with the official atomic time scale CET (Central European Time) with a transmitter power of 50 kW at a frequency of 77.5 kHz. Similar time signal transmitters exist in other countries for the transmission of time information on a long wave carrier frequency in the range between 40 to 120 kHz. All countries using such transmitters transmit the time information as a telegram having a duration of exactly 1 minute.
FIG. 1 shows an encoding scheme A referred to as a telegram. This telegram A represents the encoded time information transmitted by the German time signal transmitter DCF-77. The encoding scheme or telegram A comprises 59 bits, whereby one bit corresponds to one second of a time frame. Thus, within the duration of 1 minute a so-called time signal telegram can be transmitted. This telegram comprises binary encoded information, particularly information regarding time and date. The first fifteen bits B comprise a general encoding which, for example, may contain operational information. The next 5 bits C contain general information. Thus, the letter R designates the antenna bit. A1 designates an announcement bit for the transition from the Central European Time (CET) to a Central European Summer Time (CEST) and back again. Bits Z1 and Z2 designate time zone bits. Bit A2 designates an announcement bit for a leap second and bit S designates a start bit for the beginning of the encoded time information. Starting with bit 21 and up to bit 59 these bits transmit the time and data information with a BCD code, whereby the data are respectively relevant for the next following minute. The bits in the area D contain information regarding the minute. The bits in the area E contain information regarding the hour. The bits in the area F contain information regarding the calendar day or date. The bits in the area G contain information regarding the day of the week. The bits in the area H contain information regarding the month. The bits in the area I contain information regarding the calendar year. These informations are provided in a bit-by-bit fashion in an encoded form. So-called testing bits P1, P2, P3 are provided respectively at the ends of the areas D, E and I. The sixtieth bit of the telegram is not designated and serves for indicating the beginning of the next time frame. The letter M designates the minute marker and thus the beginning of a time signal telegram.
The structure and the bit allocation of the encoding scheme or telegram A of FIG. 1 for transmitting of time signals is generally known and described, for example in an article by Peter Hetzel, “Time Information and Normal Frequency” in the Publication “Telecom Practice”, Vol. 1, 1993.
The time signal information is transmitted with the aid of individual second markers amplitude modulated onto a carrier. The modulation comprises a reduction X1, X2 or an increase of the carrier signal X at the beginning of each second. In the case of the German time signal transmitter DCF-77 the transmitted time signals are modulated onto the carrier amplitude at the beginning of each second, with the exception of the fifty-ninth second, within each minute. For example, reducing the carrier amplitude for 0.1 second represents X1, reducing the carrier amplitude for 0.2 seconds represents X2. The amplitude reduction amounts to about 25% down from the amplitude peak. These amplitude reductions X1, X2 of different time durations define respective second markers or data bits in encoded form which are decoded in a time signal receiver. These different time durations of the second markers serve for the binary encoding of the clock time and date. Second markers X1 of a duration of 0.1 seconds correspond to the binary “O” and time markers X2 with a duration of 0.2 seconds correspond to the binary “1”. The absence of the sixtieth second marker announces the next following minute marker. An evaluation of the time information transmitted by the time signal transmitter may then be performed in combination with the respective second.
FIG. 2 illustrates a portion of an example of an amplitude modulated time signal. However, the evaluation of the precise time and the precise date is only possible if the fifty-nine second bits of a minute are recognized unambiguously so that a logic “0” or a logic “1” may be allocated to each of these respective second markers.
Frequently, noise signals are superimposed on received time signals which causes a problem because noise signals may disturb the time signal accuracy. Noise signals may, for example, be generated by electromagnetic radiations emanating from electric and electronic equipment present in the transmission range between a time signal transmitter and a time signal receiver. Even electrical components within the time signal receiver itself may cause noise. If the noise impulses are present within the range of a second impulse, such noise signals may cause a short duration increase in the signal level of the time signal. In order to demodulate the time signals including noise signals superimposed on the time signals the latter are scanned and converted into discrete values of a digital signal. This digital signal has a low logic level when the amplitude of the time signal is reduced. The signal has a high logic level corresponding to the nominal level when the amplitude is not reduced. If the duration of a noise impulse and thus of a corresponding amplitude change in the time signal is within the range of the duration of a scanning frequency cycle, then short duration fluctuations occur in the demodulated time signal. These fluctuations which are typically substantially shorter than the second impulses that are provided or predetermined by the telegram of the time signal. Problems occur, when time signals with such fluctuations are decoded. As a result, an increased computer effort and expense is necessary for gaining the time information and for evaluating the gained information in order to be able to distinguish between amplitude reductions determined by the time protocol of the time signal on the one hand and noise caused amplitude changes such as reductions on the other hand.
Another problem is seen in the fact that conventional receivers for radio-controlled clocks have a tendency to switch over too early to the nominal signal level of the time signal when the second impulses are very long. Such premature switch-over is undesirable since the second impulse or the respective amplitude reduction of the time signal is not yet completed.
The two above mentioned problems lead to a falsification of the duration of a second impulse or rather of the respective amplitude reduction of the time signals that needs to be evaluated. However, second impulses that are not falsified are absolutely necessary for a proper decoding of the information contained in the time signal. Falsified second impulses can lead to a faulty decoding of the time signal which means that at least one data bit of the minute protocol has been incorrectly evaluated. As a result, the time derived from the time signal would not be correct anymore.
The following references provide a general background information for radio-controlled clocks and receiver circuits for receiving time signals. These references are: DE 198 08 431 A1, DE 43 19 946 A1, DE 43 04 321 C2, DE 42 37 112 A1 and DE 42 33 126 A1. With regard to information retrieving and information processing of time information from time signals, reference is made to the following German Patent Publications DE 195 14 031 C2, DE 37 33 965 C2, and EP 042,913 B1.